I suggest you use Dupzilla (www.cygtools.com). The reason is suggest it is that it searches for duplicate files on your PC but give you some extra features.
1. It protects your "System" folders like C:\Windows, Program Files, etc. You don't' want to mess with those folders because if you delete some necessary file from these folders then it might corrupt your windows or other programs. Dupzilla automatically exculdes these folders from scan.
2. It lets you filter your searches by type. For example you could search for only Music/Video files, mp3, wmv, etc. This feature is good if you are only concerned about music files although the program can search for other types of duplicate files as well.
3. It lets you filter duplicate files by size. For example you could say that you only want to see duplicate files larger then 5KB or some other limit if you don't want to look at hundreds of small duplicates which don't really consume much space.
4. It lets you save a *.dup file which saves all your search settings (folders, types of files to search, size filter, etc.). Then you just double click that .dup file and it searches everything for you. So if you are doing the search on a regular basis (let's say monthly) then you don't have to make all the selections again.
5. Best of all it is free right now. There website says it is for a limited time. Not sure how long that deal is going to last.